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It sounds a bit weird to say, considering we’ve all lived our entire lives on this planet, but scientists still only have a pretty basic understanding of the inner workings of Earth. We know the fundamental structure of the planet and its core, but there are still plenty of unanswered questions about how the planet’s core and mantle change over time and what movements are taking place deep beneath our feet. Now, the discovery of a mineral trapped within a diamond is offering some important clues.

The key to the discovery is a large diamond dug up from the Cullinan Mine in South Africa. The precious stone has a specific mineral, called calcium silicate perovskite, trapped inside of it. It’s a mineral that is not found on Earth’s surface but is incredibly abundant in the lower parts of the planet’s mantle. How it got way up to the crust is the subject of a new paper published in Nature.

“Nobody has ever managed to keep this mineral stable at the Earth’s surface,” explains Graham Pearson, of the University of Alberta, and co-author of the study. “The only possible way of preserving this mineral at the Earth’s surface is when it’s trapped in an unyielding container like a diamond. Based on our findings, there could be as much as zettatonnes (1021) of this perovskite in deep Earth.”

The fact that the diamond has such a mineral trapped inside of it means that it likely formed many hundreds of miles below the surface. That’s significant because it means that the Earth recycles parts of its crust, such as oceanic plates, back into its mantle over long periods of time. This type mechanism had been theorized in the past but this newly discovered gem has provided the first hard evidence that it might indeed be true.

“It’s a nice illustration of how science works,” Pearson said in a statement. “That you build on theoretical predictions — in this case, from seismology — and that once in a while you’re able to make a clinching observation that really proves that the theory works.”